Implantable stimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) techniques, which directly stimulate the spinal cord tissue of the patient, have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of spinal cord stimulation has begun to expand to additional applications, such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients. Occipital Nerve Stimulation (ONS), in which leads are implanted in the tissue over the occipital nerves, has shown promise as a treatment for various headaches, including migraine headaches, cluster headaches, and cervicogenic headaches. In recent investigations, Peripheral Stimulation (PS), which includes Peripheral Nerve Field Stimulation (PNFS) techniques that stimulate nerve tissue directly at the symptomatic site of the disease or disorder (e.g., at the source of pain), and Peripheral Nerve Stimulation (PNS) techniques that directly stimulate bundles of peripheral nerves that may not necessarily be at the symptomatic site of the disease or disorder, has demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation.
Each of these implantable stimulation systems typically includes an electrode lead implanted at the desired stimulation site and neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the electrode lead or indirectly to the electrode lead via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation lead(s) to stimulate or activate a volume of neural tissue. In particular, electrical energy conveyed between at least one cathodic electrode and at least one anodic electrode creates an electrical field, which when strong enough, depolarizes (or “stimulates”) the neurons beyond a threshold level, thereby inducing the firing of action potentials (APs) that propagate along the neural fibers.
Stimulation energy may be delivered to the electrodes during and after the lead placement process in order to verify that the electrodes are stimulating the target neural elements and to formulate the most effective stimulation regimen. The regimen will dictate which of the electrodes are sourcing current pulses (anodes) and which of the electrodes are sinking current pulses (cathodes) at any given time, as well as the amplitude, duration, rate, and burst rate of the stimulation pulses.
The stimulation regimen will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. In the case of SCS and PS, such a therapeutic benefit is accompanied by “paresthesia,” i.e., a tingling sensation that is effected by the electrical stimuli applied through the electrodes.
The stimulation system may further comprise a handheld remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon. If the IPG contains a rechargeable battery, the stimulation system may further comprise an external charger capable of transcutaneously recharging the IPG via inductive energy.
It is sometimes desirable to use a hybrid stimulation regimen that combines multiple types of stimulation to treat one or more disease conditions. For example, despite many reports of success in relieving radicular pain in the lower extremities and buttocks with SCS, physicians often report difficulties with achieving and maintaining adequate pain control over the long term for patients experiencing axial low back pain, especially for patients experiencing chronic pain due to failed back surgery syndrome (FBSS). Thus, SCS is often inadequate in relieving both the back and leg pain components. However, for patients with chronic low back pain as a result of FBSS, it has been shown that PNFS is extremely effective in reducing pain and enabling patients to resume their normal activities (see Richard M. Paicius, M. D., et al., Peripheral Nerve Field Stimulation for the Treatment of Chronic Low Back Pain: Preliminary Results of Long-Term Follow-up: A Case Series, Neuromodulation, Volume 10, Number 3, 2007, and Jason P. Krutsch, M. D., et al., A Case Report of Subcutaneous Peripheral Nerve Stimulation for the Treatment of Axial Back Pain Associated with Postlaminectomy Syndrome,” Neuromodulation, Volume 11, Number 2, 2008).
Thus, the use of PNFS as an adjunct to SCS may overcome the limitations of SCS, and may be valid option for the treatment of patients whose pain is severe both in the axial back and legs, with the SCS component targeting the radicular pain, and the PNFS more directly and completely relieving the lower back pain. Based on case studies performed on patients, it has been found that a combination of SCS and PNFS to control lower back and leg pain is, indeed, more effective than either modality alone. Thus, it can be concluded that PNFS may be used in combination with SCS as a safe and effective alternative treatment for patients with chronic low back and leg pain (see Clifford A. Bernstein, M. D., et al., Spinal Cord Stimulation in Conjunction with Peripheral Nerve Field Stimulation for the Treatment of Low Back and Leg Pain: A Case Series, Neuromodulation, Volume 11, Number 2, 2008).
These hybrid stimulation systems oftentimes utilize a single IPG that delivers the stimulation energy to support the different stimulation regimens, each of which requires a vastly different programming technique and associated programming interface. For example, in SCS, a large number of tightly spaced electrodes are implanted within the epidural space of the patient to stimulate the spinal cord tissue within a high resolution electrical field, whereas in PNFS, a small number of widely spaced apart electrodes are implanted in the subcutaneous tissues of a peripheral region, such as the lower back region, to directly stimulate the peripheral field (i.e., the region of the affected nerves, the cutaneous afferents, or the dermatomal distribution of these nerves, which then converge back within the spinal cord) in the region of pain. Thus, different programming strategies must typically be employed for a single hybrid stimulation system. In order to best program these hybrid stimulation systems, it would be desirable to quickly and automatically determine the anatomical regions of the patient in which the stimulation leads are implanted, so that the proper stimulation regimens can be applied to these anatomical regions.
It would also be desirable to determine the depth of implantation for a stimulation lead, since depending on the anatomy of the targeted stimulation region and the implant depth of the stimulation lead, the patient may experience different sensations during the stimulation. For example, stimulation lead depth plays an important role in PNFS. If the stimulation lead depth is too superficial, the patient may feel discomfort or painful sensations resulting from A-delta or C fiber action at a lower amplitude, which reduces the therapeutic stimulation range for the targeted sensory fibers. If the stimulation lead is too deep, muscle activation may result during stimulation, again limiting the therapeutic stimulation range for the target sensory fibers. Conventional techniques, such as fluoroscopy, can be used to determine the stimulation lead depth, but requires bulky instruments and involves ionized radiation.